This application is based upon and claims the benefit of priority from the prior Japanese patent application No. 11-280010, filed Sep. 30, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a linear amplifier and a radio communication apparatus using the linear amplifier, and more particularly to a linear amplifier with a gain varying function using bipolar transistors and a radio communication apparatus using this linear amplifier.
When a modulated wave signal whose amplitude varies greatly is dealt with, a conventional amplifier is used in the range of values far backed off from the saturated output to prevent gain compression from limiting the amplitude. Thus, it is impossible to use the amplifier in its high efficiency region.
As for a method of using such an amplifier as a linear amplifier up to almost the saturated output point, various linearizers (see FIG. 22) for canceling the nonlinear characteristics have been designed. Typical linearizers include a system for producing the reverse distortion component in the input signal beforehand and a system for varying the gain of an amplifier according to a variation in the amplitude of the input signal and equalizing the gain in the gain compression region to that in the linear region.
In such linearizers, their added circuits are complex and consume a lot of power. They can be used in the last-stage amplifier in a base station that amplifies high power. They, however, have not been put to practical use in the field of about 1-W-output amplifiers for use with portable telephones, because they have many disadvantages in that the total efficiently hardly improves and the size of the component parts is large.
On the other hand, a high-efficiency amplifier that presents a linear characteristic in a wide dynamic output range is realized by setting the bias condition to class B amplification to reduce the idle current. Actually, however, since the distortion becomes greater due to the nonlinear characteristic of the mutual conductance of elements, it is impossible to set the bias condition to heavy class B amplification by which the idle current is reduced extremely. Therefore, it is desirable that, in a low-output level region where the distortion is small, the bias should be varied in such a manner that the bias current is reduced extremely.
Since in a class-B amplifier using bipolar transistors, the average collector current increases according to the output level, the bias circuit has to supply a sufficient current equivalent to an increase in the average base current accordingly. For this reason, in a current-mirror circuit for supplying the base current via the emitter-follower and others, a bias circuit for supplying a voltage with a reduced impedance shown in FIG. 20 or 21 is generally used.
In the bias circuit shown in FIG. 20, the collector of a transistor Q21 is connected to one end of a resistance R21 via a terminal 11. A control voltage of Vcont is applied to the other end of the resistance R21 (on a terminal 2xe2x80x2 side). The base of the transistor Q21 is connected to the emitter of a transistor Q3xe2x80x2. The base of the transistor Q3xe2x80x2 is connected to the terminal 11. A power-supply voltage of vcc is applied to the collector of the transistor Q3xe2x80x2. The emitter of the transistor Q21 is grounded. The emitter of the transistor Q3xe2x80x2 is not only rounded via a resistance R3xe2x80x2 but also connected to the base of a terminal 4xe2x80x2 amplification transistor.
In the bias circuit shown in FIG. 21, transistors Q4xe2x80x2, Q5xe2x80x2, are used in place of the transistor Q2xe2x80x2. Specifically, the emitter of the transistor Q4xe2x80x2 is connected to the collector of the transistor Q5xe2x80x2 and the collector of the transistor Q4xe2x80x2 is connected to a resistance R21. The emitter of the transistor Q5xe2x80x2 is grounded. The base and collector of the transistor Q4xe2x80x2 are short-circuited. The base and collector of the transistor Q5xe2x80x2 are short-circuited. These transistors function as diodes.
With such a circuit, when an attempt is made to reduce the current in proportion to the voltage, fluctuations in the bias current due to temperature changes cannot be compensated for sufficiently unless its control voltage Vcont is made very high. When the control voltage is made high, this becomes a serious problem in a system with a low control voltage, such as a portable telephone. Particularly in a system that has to operate linearly at an output level in a wide dynamic range, such as CDMA (Code Division Multiple Access), both the current and the gain are too large at a low output level at the bias point that satisfies the distortion characteristic at the high output level.
As explained above, when a conventional linear amplifier attempts to realize a linear operation in a high-efficiency operation, it requires a linearizer as shown in FIG. 22. The system using a linearizer, however, does not produce much effect in a 1-W-class amplifier, such as an amplifier for a portable telephone in terms of compactness and total efficiency. When such an amplifier is operated in the linear region, it is eventually used at 20 to 30% lower efficiency.
Furthermore, when an attempt is made to control the bias current in a wide range to reduce the current during the low output, it is necessary to raise the control voltage or sacrifice temperature compensation. This makes it difficult to lower the voltage or reduce the power consumption in a system that has to operate linearly at the output level in a wide dynamic rage, such as CDMA.
The object of the present invention is to provide a linear amplifier with a bias circuit which is capable of realizing a high-efficiency operation during high output, while maintaining a linear operation, and of reducing the bias voltage with a low control voltage during low output without increasing the number of circuits and the drawn current as compared with a conventional equivalent circuit, and a radio communication apparatus using the linear amplifier.
According to a first aspect of the present invention, there is provided a linear amplifier comprising: a first current-mirror circuit including a first transistor whose base and collector are short-circuited for diode connection, whose collector is connected via a first resistance to a power-supply terminal, and whose emitter is grounded; a second current-mirror circuit including a second transistor whose collector and base are connected to power-supply terminals; and an amplification transistor whose emitter is grounded, wherein the base of the first transistor and the emitter of the second transistor are connected to the base of the amplification transistor.
According to a second aspect of the present invention, there is provided a linear amplifier including two or more stages of amplification transistors whose emitters are grounded, the linear amplifier comprising: a first current-mirror circuit including a first transistor whose base and collector are short-circuited for diode connection, whose collector is connected via a first resistance to a power-supply terminal, and whose emitter is grounded; and a second current-mirror circuit including a second transistor whose collector and base are connected to power-supply terminals, wherein the base of the first transistor and the emitter of the second transistor are connected to the base of a transistor at an earlier stage than the last stage of the amplification transistors.
In the linear amplifiers according to the first and second aspects of the present invention, it is desirable that they should meet the following configuration requirements:
(1) The collector of the first transistor and the base of the second transistor are connected to a common power supply.
(2) The base of the second transistor is connected via a second resistance to a power-supply terminal.
(3) The collector of the first transistor is connected via the first resistance to the base of the second transistor and the base of the second transistor is connected to the power-supply terminal of the first transistor.
(4) The first current-mirror circuit has a higher impedance than the second current-mirror circuit when viewed from the amplification transistor side.
(5) The base of the second transistor is grounded via a plurality of diodes connected in series.
(6) Each of the plurality of diodes is composed of a transistor whose base and collector are short-circuited for diode connection.
(7) The second current-mirror circuit includes a third transistor whose emitter is grounded, whose base is connected to the base of the first transistor, and whose collector is connected to the base of the second transistor.
(8) The first current-mirror circuit applies a bias to the base of the amplification transistor when the output level of the amplification transistor is low, and the second current-mirror circuit applies a bias to the base of the amplification transistor when the output level of the amplification transistor is high.
(9) The first and second current-mirror circuits are so set that the distortion of modulation in the amplification characteristic of the amplification transistor at an earlier stage than the amplification transistor at the last stage cancels the distortion of modulation in the amplification characteristic of the amplification transistor at the last stage.
According to a third aspect of the present invention, there is provided a linear amplifier comprising: a first current-mirror circuit including a first transistor whose base and collector are short-circuited for diode connection, whose collector is connected to a first resistance to a variable-power-supply terminal, and whose emitter is grounded, and a second current-mirror circuit including a second transistor whose collector is connected to a power-supply terminal and whose base is connected to a variable-power-supply terminal; and an amplification transistor whose emitter is grounded, wherein the base of the first transistor and the emitter of the second transistor are connected to the base of the amplification transistor.
According to a fourth aspect of the present invention, there is provided a linear amplifier including two or more stages of amplification transistors whose emitters are grounded, the linear amplifier comprising: a first current-mirror circuit including a first transistor whose base and collector are short-circuited for diode connection, whose collector is connected via a first resistance to a variable-power-supply terminal, and whose emitter is grounded; and a second current-mirror circuit including a second transistor whose collector is connected to a power-supply terminal and whose base is connected to a variable-power-supply terminal, wherein the first current-mirror circuit and the second current-mirror circuit are provided for each of the two or more stages of amplification transistors, and the base of the first transistor and the emitter of the second transistor are connected to the base of the corresponding one of the amplification transistors.
In the linear amplifiers according to the third and fourth aspects of the present invention, it is desirable that they should meet the following configuration requirements:
(1) The collector of the first transistor and the base of the second transistor are connected to a common power supply.
(2) All the variable-power-supply terminals in the first current-mirror circuit and the second current-mirror circuit provided for each stage of the two or more stages of amplification transistors are a common variable-power-supply terminal.
(3) The base of the second transistor is connected via a second resistance to a variable-power-supply terminal.
(4) The collector of the first transistor is connected via the first resistance to the base of the second transistor and the base of the transistor is connected to the variable-power-supply terminal of the first transistor.
(5) The first current-mirror circuit has a higher impedance than the second current-mirror circuit when viewed from the amplification transistor side.
(6) The base of the second transistor is grounded via a plurality of diodes connected in series.
(7) Each of the plurality of diodes is composed of a transistor whose base and collector are short-circuited for diode connection.
(8) The second current-mirror circuit includes a third transistor whose emitter is grounded, whose base is connected to the base of the first transistor, and whose collector is connected to the base of the second transistor.
(9) The power-supply terminal to which the collector of the second current-mirror circuit is connected is a fixed-potential power-supply terminal.
(10) The gain of the amplification transistors is varied by controlling the power-supply voltage at the variable-power-supply terminal.
According to a fifth aspect of the present invention, there is provided a linear amplifier including two or more stages of amplifiers composed of emitter-grounded transistors, the linear amplifier comprising: a bias circuit for supplying a direct-current bias base voltage to the amplification transistor at each stage of the amplifiers; a first current-mirror circuit which includes a first transistor whose base and collector are short-circuited for diode connection, determines the base voltage with the first transistor, and has a high output impedance; and a second current-mirror circuit which includes a second transistor whose base and collector are connected with each other via an emitter follower transistor, determines the base voltage with the second transistor, and has a low output impedance, and which is connected in parallel with the first current-mirror circuit.
According to a sixth aspect of the present invention, there is provided a linear amplifier comprising: a first current-mirror circuit including a diode element whose anode is connected via a first resistance to a power-supply terminal and whose cathode is grounded; a second current-mirror circuit including a transistor element whose collector and base are connected to power-supply terminals; and an amplification transistor whose emitter is grounded, wherein the anode of the diode element and the emitter of the transistor element are connected to the base of the amplification transistor.
According to a seventh aspect of the present invention, there is provided a linear amplifier including two or more stages of amplification transistors whose emitters are grounded, the linear amplifier comprising: a first current-mirror circuit including a diode element whose anode is connected via a first resistance to a power-supply terminal and whose cathode is grounded; and a second current-mirror circuit including a transistor element whose collector and base are connected to power-supply terminals, wherein the anode of the diode element and the emitter of the transistor element are connected to the base of an amplification transistor at an earlier stage than the last stage of the amplification transistors.
According to an eighth aspect of the present invention, there is provided a linear amplifier comprising: a first current-mirror circuit including a diode element whose anode is connected via a first resistance to a variable-power-supply terminal and whose cathode is grounded; a second current-mirror circuit including a transistor element whose collector is connected to a power-supply terminal and whose base is connected to a variable-power-supply terminal; and an amplification transistor whose emitter is grounded, wherein the anode of the diode element and the emitter of the transistor element are connected to the base of the amplification transistor.
According to a ninth aspect of the present invention, there is provided a linear amplifier including two or more stages of amplification transistors whose emitters are grounded, the linear amplifier comprising: a first current-mirror circuit including a diode element whose anode is connected via a first resistance to a variable-power-supply terminal and whose cathode is grounded; and a second current-mirror circuit including a transistor element whose collector is connected to a power-supply terminal and whose base is connected to a variable-power-supply terminal, wherein the first current-mirror circuit and the second current-mirror circuit are provided for each of the two or more stages of amplification transistors, and the anode of the diode element and the emitter of the transistor element are connected to the base of the corresponding one of the amplification transistors.
According to a tenth aspect of the present invention, there is provided a linear amplifier including two or more stages of amplifiers composed of emitter-grounded transistors, the linear amplifier comprising: a bias circuit for supplying a direct-current bias base voltage to the amplification transistor at each stage of the amplifiers; a first current-mirror circuit which includes a diode element, determines the base voltage with the diode element, and has a high output impedance; and a second current-mirror circuit which includes a second transistor whose base and collector are connected with each other via an emitter follower transistor, determines the base voltage with the second transistor, and has a low output impedance, and which is connected in parallel with the first current-mirror circuit.
Furthermore, there is provided a radio communication apparatus using any one of the linear amplifiers of the invention described above.
In the radio communication apparatus according to the present invention, it is desirable that the power-supply voltage at the variable-power supply terminal should be set low when the output level is low and thereby the gain of the amplification transistor at the last stage decrease. In a high output operation, the first current-mirror circuit applies a bias when the input level is low, and the second current-mirror circuit supplies a bias when the input level is high, in at least one amplification transistor other than one at the last stage and its bias circuit. This enables the AMxe2x80x94AM characteristic of the amplification transistor to cancel the AMxe2x80x94AM characteristic of the amplification transistor at the last stage. Therefore, even when the last stage is in a high-efficiency operation, the distortion of intermodulation can be decreased.
Furthermore, a linear operation at high efficiently and a smaller drawn current in low output can be realized by operating the first and second current-mirror circuits according to the control voltage that determines the input power level or bias current.
Moreover, in use at low power level, a smaller drawn current can be achieved without sacrificing distortion by lowering the control voltage to decrease the idle current. In addition, using the linear amplifier as the amplifier at the last stage of a CDMA terminal makes it possible to achieve a smaller drawn current in a wide output dynamic range and simplify the system.
For example, the terminal for the control voltage applied via a resistance functioning as the current source for the first current-mirror circuit of the amplifier is connected to the terminal for the control voltage applied via a resistance functioning as the current source for the second current-mirror circuit. With this connection, the first current-mirror circuit supplies the bias voltage when the control voltage is equal to or lower than twice the on voltage, the base-emitter voltage Vbe of the transistor, whereas the first and second current-mirror circuits supply the bias voltage when the control voltage is higher than the on voltage. This makes variations in the gain in the wide control voltage range compatible with compensation for its temperature changes. Furthermore, a smaller drawn current operation in low-gain, low-output use is possible by varying the control voltages in the bias circuits at the amplification stages including the last stage at the same time.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.